John Thomas Flavin was born in Albany, California, on May 7, 1942. He attended El Cerrito High School in California, where he participated in baseball, basketball, and football and graduated in 1959. Flavin later attended the University of Southern California. He was a left-handed batter and pitcher, standing 6 feet 2 inches tall and weighing 208 pounds during his playing career.
Flavin graduated from El Cerrito High School in 1959 and later attended the University of Southern California in Los Angeles. His high-school athletic participation included baseball, basketball, and football, and his subsequent collegiate education preceded his entry into professional baseball.
Flavin entered professional baseball in 1960 in the Cincinnati Reds organization, beginning with Palatka in the Florida State League. He advanced to Topeka in 1961 and divided the 1962 season between Macon and Triple-A San Diego, posting a 12–2 record for San Diego. He remained with San Diego in 1963 before the Cincinnati organization sold his contract to the Chicago Cubs in April 1964. With Triple-A Salt Lake City, he won seven of twelve decisions and was recalled to Chicago in August. Flavin made his major-league debut for the Cubs on August 25, 1964, starting against the Houston Colt .45s at Colt Stadium. He pitched two innings, allowing six hits, two walks, a wild pitch, and five earned runs in a 5–4 defeat. He made four additional appearances in relief, finishing his major-league career with five games, one start, a 0–1 record, a 13.50 earned-run average, 4.2 innings, and five strikeouts. After missing the 1965 season, he returned to the Cubs organization with Triple-A Tacoma in 1966, his final professional season.
Physical Characteristics: Flavin was listed as 6 feet 2 inches tall and 208 pounds. He batted left-handed and threw left-handed.
